Polishing a work, e.g., semiconductor wafer, is performed by steps of: pressing a surface of the work to be polished onto a surface of a polishing pad adhered on a polishing plate; supplying slurry onto the polishing pad; and rotating the polishing plate.
However, by polishing many works, the polishing pad is gradually clogged, and a polishing rate is lowered. To solve this problem, the polishing rate is recovered by dressing the surface of the polishing pad, by using a dressing grindstone, after polishing works a prescribed number of times (see PTL 1: JP-A-2001-260001).
PTL 1 discloses a method for flattening a semiconductor wafer by using a dressing rate measuring unit for measuring a polishing rate of a polishing pad, which is varied with progress of polishing, and a surface property measuring unit for measuring a surface property of the polishing pad. In the method, a dressing condition is controlled so as to keep the dressing rate, which seriously influences on a scratch density, within a prescribed management value range which has been previously stored in a database.
In PTL 1, the surface property of the polishing pad is measured by an image processing manner or a manner of measuring a reflection rate.
In the image processing manner, the surface of the polishing pad is irradiated by a light projector, an image of the irradiated part is extracted by a CCD camera, the image is processed, and an area ratio of a flat part of the polishing pad formed by clogging is measured. On the other hand, in the manner of measuring a reflection rate, a laser beam is applied to the surface of the polishing pad, the reflected laser beam is received by a light receiving unit, and the surface property of the polishing pad is measured on the basis of light quantity variations of the received laser beam.
In PTL 1, the surface property of the polishing pad is measured and the surface thereof is dressed while polishing the work, so that there is an advantage that dressing the polishing pad can be suitably performed according to the momentarily varying surface property of the polishing pad.
However, in PTL 1, the surface property of the polishing pad is measured while polishing the work, so an image different from the actual one will be obtained or an unclear image will be obtained due to polishing dusts and slurry (e.g., cloudy liquid). Therefore, precise data of the surface property of the polishing pad cannot be obtained.
Further, the surface property of the polishing pad cannot be correctly obtained, so it is often obtained on the basis of operators' experiences, even now. This problem impedes automatizing the polishing process and making intelligent the same.